The present invention relates to a dashboard or instrument panel for vehicles such as automobiles, and more particularly to an illumination method for an electronic instrument panel using a color liquid crystal display.
As the development in electronics toward various instruments advances, electronic instrument panels using liquid crystal display elements have recently widely been used for automobiles and the like.
In the early stage of the development, such liquid crystal display elements could only operate to turn on and off. However, recently elements capable of changing display color, i.e., so-called color liquid crystal display elements have widely been used in practice.
Among such color liquid crystal display elements, various types are known. For example, in the Official Gazette of Japanese Patent Unexamined Pubication No. 56-21182 a guest-host type liquid crystal capable of effecting a multiple color display is disclosed, and in the Official Gazette of Japanese Patent Unexamined Publication No. 58-150935 a smectic type liquid crystal capable of effecting a multiple color display is disclosed.
Liquid crystal display elements are passive elements with respect to light so that a so-called background light source is required at the back of the liquid crystal display elements, for example, of the transmission type, which are commonly used for electronic instrument panels. By changing the color of the background light, a multiple color display is obtained which is disclosed in the Official Gazette of Japanese Patent Unexamined Publication no. 58-88778. Other various methods for a multiple color display have been proposed heretofore, for example, a multiple color display method with moving filters has been proposed. Guest-host and smectic type liquid crystals are however expensive as compared with common TN type liquid crystals. Thus, it is difficult to manufacture a low cost instrument panel.
The method employing the color change of the background light also encounters difficulties in obtaining a partial color change on the display section.
In view of the above, there is disclosed in the Official Gazette of Japanese Patent Unexamined Publication No. 58-147781 a color liquid crystal wherein primary color stripe filters are provided for each portion of the display section, e.g., each segment of the seven-segment display device. With this method, a multiple color display can be realized by using inexpensive TN type liquid crystals. Moreover, it is advantageous in that various kinds of colors can be produced by mixing primary colors.
The use of such color liquid crystals with stripe filters however is associated with a drawback that the ability of visual recognition is degraded due to the change of a display area for each color and hence due to the change of luminance for each color at the display section.
The more detailed description for the above will be given with reference to the accompanying drawings. In FIG. 2, the display segments of a primary color stripe type color liquid crystal are shown. Each of the seven segments is divided minutely into 3N stripes. Three groups of N stripes constitute the display section. R (red), G (green) and B (blue) stripe filters are respectively provided for each stripe of the same group. Three colors R, G and B and a desired number of other colors to be produced by mixing R, G and B are possible. For example, seven colors in all (red, green, blue, cyan, magenta, yellow and white) can be produced.
In this case, the display areas occupied by respective colors are different from each other as shown in FIG. 3. Particularly, assuming that the display area for a white color is 1, to be obtained by rendering all of the corresponding R, G and B stripe filters active, then each display area for cyan, magenta and yellow colors is 2/3 and each display area for R, G and B is 1/3. Therefore, the display luminance varies with color.